Irrigation 101

Brian E. Vinchesi

If the precipitation rate varies significantly over the area being
irrigated, uniformity is poor; a precipitation rate that is nearly equal
throughout the area provides good uniformity. For good uniformity, you need
to match the flow of the sprinkler to the area it is watering. This is
known as "matched precipitation."

When you use spray sprinklers, matched precipitation is automatic. The
nozzles, which are fixed arcs, use an amount of water proportional to the
area they cover. For example, if a full-circle (360 degrees) sprinkler
throws 15 feet and uses 3.7 gpm, a half-circle sprinkler (180 degrees)
still throwing 15 feet would be expected to use 1.85 gpm, and a
quarter-circle 0.93 gpm. All the precipitation rates would be theoretically
equal at 1.58 inches per hour.

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Rotary sprinklers are different. Nozzles usually are interchangeable, and
you can choose a nozzle from those available from the manufacturer. All the
sprinklers could have the same nozzle regardless of the area covered. For
example, a sprinkler with a #5 nozzle throws 35 feet using 2.4 gpm. There
are five sprinklers, all with the #5 nozzle. Two of the sprinklers are
full-circle (360 degrees), one is a quarter-circle (90 degree) sprinkler
and two are half-circle (180 degrees) sprinklers. The precipitation rate is
different for each area-the amount of water discharged stays the same for
each sprinkler even though the areas covered by each vary. The
quarter-circle sprinkler applies four times the amount of water as the
full-circle sprinkler.

To uniformly irrigate rotary sprinklers, you need to use different nozzles
or use separate zones for arced sprinklers. For example, a rotary
sprinkler's available nozzles might be 1.5, 3.0 and 6.0 gpm. You would use
the 1.5 gpm nozzle for the quarter circles, 3.0 gpm for the half circles
and 6.0 for the full circles to get matched precipitation.

Water in zones

Divide all irrigation systems into zones, because the amount of water you
need to cover an entire area at one time is probably not available,
especially at a residence. Zoning makes the system more economical-piping
is smaller and installation is easier. The zone size is strictly dependent
on the amount of safe flow available at the water source. The more water
available, the larger the zones can be.

In establishing zones, consider factors other than just the available flow.
You should zone similar sprinklers together so that precipitation rates are
equal. Zone sunny and shaded areas separately to address the varying water
requirements of plant material. Zone turf and landscaped areas separately.
They need different watering schedules because of their varying root-zone
depths and water requirements. Different soil types also require different
zoning, due to their water infiltration rates.

Zones are turned on and off using low-voltage electric valves. The required
size of a zone valve is based on the amount of water flowing through it and
its friction loss. Many times, the electric valve is smaller than the
piping on which it is installed. You can find charts for valve sizing in
manufacturers' catalogs.

Electric zone valves, other than inexpensive residential valves, will have
flow control. This feature enables you to use a handle to adjust the flow
of water and, as a result, the pressure.

You can buy valves with a pressure-regulating feature. Sprinklers rarely
deliver the exact pressure that their design calls for, and many sites have
pressure that is too high. A pressure-regulator option will lower the
pressure immediately after the valve to a set value. However, a valve
cannot produce more pressure than it receives.